If you have an "always-on" at the front, that essentially is your input buffer, because it's the first thing the passive guitar sees. Whether it is designed to buffer, is good at it, or whatever, that's essentially the input buffer. Now, if you have another buffer later, I guess you'd call that an interstitial buffer. This can still be handy, if you have a pedal like a simple boost or fuzz that has a high output impedance. Remember, the higher the output (also called source) impedance, the more it can interact with the input (or load) impedance of the next stage. Here's an example - let's say you have a simple boost with an output impedance of 10K. You plug it into an overdrive with lower input impedance, like 100K. There's a 10% loss happening, because it's essentially a voltage divider with a 10K resistor and a 100K resistor.
In this case an interstitial buffer might save you that 10%. Importantly, this is frequency-dependent because impedance is not purely resistive. So you'll probably notice some brightness coming back before it feels like more level in general. It's important to note though, that layering extra stages of any kind, buffering amplifier or gain amplifier or whatever, can have a detrimental effect. When all other things are equal, the shorter and simpler signal path will generally be less noise, have better phase response and sound better.
If your "always-on" has created any loss itself, because of its input impedance, or headroom, or other aspect, that information is gone. That's why it's important to have the loss-preventing stages first.
Now, if the output of your always-on is has low output impedance, like EUNA is about 10 ohms, the load doesn't really matter, until it gets to the point of current drive limiting. EUNA will handle down to a 600 ohm load, so anything basically. You can turn anything on or off, or have almost any conceivable load, and it will be fine with no other devices needed.
Sometimes this "always on" is correcting for loss of the next stage - whether people know it or not. If your favorite overdrive has 100K input impedance, and that is usually at the front, and you add an "always-on" compressor or boost that has a higher input impedance, you'll notice less loss, especially in the high-end. But that pedal may also add distortion, tone shaping, etc., and you may or may not want that. You also may want to use that pedal creatively, as it was probably designed. You'll find sometimes that using EUNA eliminates the need for the always-on, but not always. It gets down to preference very fast.
This is why I generally advocate for a proper input driver, which I would hope to be EUNA but many other good devices exist, so that whatever choices you make in your chain don't affect the fundamental tone in a negative way.
It frees you up to use those "always on" pedals creatively, dial them in differently for different scenarios, and in general arrange your chain more creatively. Less worry about loading and loss.